MMSET (also called WHSC1, NSD2) is a histone methyltransferase (HMT) that has been shown to methylate histone H4 Lys20 (H4K20), H3 Lys27 (H3K27) and H3 Lys36 (H3K36). Deletion of MMSET is partly responsible for the rare genetic diseases Wolf-Hirschhorn Syndrome, a multiple malformation syndrome. On the other hand, overexpression of MMSET is linked to tumorigenesis in multiple myeloma. How misregulation of MMSET contributes to the etiology of these human diseases is still not clear. Besides several reports linking it to transcriptional regulation, the cellular function of MMSET remains obscure. We have recently found that MMSET participates in the ATM-MDC1-53BP1 pathway during the DNA damage response. Specifically, MMSET constitutively interacts with 53BP1 and accumulates at sites of DNA damage. Correlating with this, H4K20 methylation, which is required for 53BP1 recruitment [4], also increases at the sites of DNA damage. Downregulation of MMSET abolishes the accumulation of 53BP1 to the sites of DNA damage. These results suggest that MMSET functions as an upstream regulator of 53BP1. In addition, following DNA damage, MDC1 interacts with MMSET in an ATM dependent manner, and MDC1 is required for the accumulation of MMSET at the sites of DNA damage, suggesting the ATM-MDC1 pathway regulates MMSET recruitment to the sites of DNA damage. Finally, we found that MMSET is overexpressed in a subset of glioblastoma lines, and overexpression of MMSET is associated with resistance to temozolomide (TMZ) and radiation (RT). Based on these preliminary findings, we hypothesize that MMSET regulates 53BP1 and the DNA damage response, and that misregulation of MMSET could affect sensitivity to chemoradiotherapy (Figure 1). To further examine this hypothesis, we propose the following Specific Aims: 1. Investigate how MMSET regulates the recruitment of 53BP1 to the sites of DNA damage and affects the DNA damage response. 2. Investigate how MMSET is regulated by the DNA damage response pathway;3. Investigate the role of MMSET in glioblastoma sensitivity to temozolomide and radiation. These studies will reveal a novel role of MMSET in cellular response to genotoxic stress. Furthermore, these studies will link MMSET to cancer prognosis and identify MMSET as a possible target for cancer therapeutics. PUBLIC HEALTH RELEVANCE: MMSET (also called WHSC1, NSD2) has been implicated in many human diseases, such as Wolf-Hirschhorn and multiple myeloma. However, the cellular function of MMSET is not clear. Here we will investigate a role of MMSET in DNA damage response and chemoradioresistance. These studies will reveal a novel role of MMSET in cellular response to genotoxic stress. Furthermore, these studies will link MMSET to cancer prognosis and identify MMSET as a possible target for cancer therapeutics.